Controlled potential microelectrode techniques—potential sweep methods

(1) Potential sweep methods: linear sweep voltammetry (LSV) and cyclic voltametry (CV).

(2) Cyclic voltammetry is a very popular technique for initial electrochemical studies of new systems and has proven very useful in obtaining information about fairly complicated electrode reactions.

(3) Signal Response

Linear sweep voltammetry

Signal Resulting i-E curve

A typical LSV response curve for the reduction

• At a potential well positive of E0´, only nonfaradaic currents flow for awhile.

• When the potential reaches the vicinity of E0´, the reduction begins and current starts to flow.

• As the potential continues to grow more negative, the surface concentration of the reactant must drop, hence the flux to the surface and the current increase.

• As the potential moves past , the surface concentration drops to near zero and mass transfer of reactant to the surface reaches a maximum rate.

• Then it declines as the depletion effect sets in.

Cyclic voltammetry

Cyclic potential sweep

Resulting cyclic

voltammogram(initial potential and switching potential)

Sweep voltammogram depends on a numb

er of factors including:

• Scan rate• Pathway of a general electrode reaction• Reaction rate of the rate-determining steps)• Chemical reactivity of the electroactive speci

es

Scan rate

• In LSV, the potential is scanned from a lower limit to an upper limit

• Unit of scan rate(υ): V/s or mV/s

• Effects of scan rate on charging current:

[( ) exp( / )ic d d s d

s

Ei C C t R C

R

In general, the electrode reaction rate is governed by rates of processes such as:

(1) Mass transfer (e.g., from the bulk solution to the electrode surface).

(2) Electron transfer at the electrode surface.(3)Chemical reactions preceding or following the

electron transfer.(4)Other surface reactions.

◆ The magnitude of this current is often limited by the inherent sluggishness of one or more reactions called rate-determining steps.

Factors affecting electrode reaction rate

Scan rate

If the scan rate is altered the current response also changes.

Rate-determining steps

0 , ,

1 1 1( )

l c l a

RTi

nF i i i

• Here we see very clearly that when i0 is much greater than the limiting currents, Rct<<Rmt,c + Rmt,a and the overpotential, even near Eeq, is a concentration overpotential. On the other hand, if i0 is much less than the limiting currents, then Rmt,c + Rmt,a<<Rct, and the overpotential near Eeq is due to activation of charge transfer.

, ,( )ct mt c mt ai R R R

Peak current and scan rate

• At 25 , ℃ ip is

* 1/ 2 1/ 20.4463p

RTi nFAC D

nF

5 3/ 2 * 1/ 2 1/ 2(2.69 10 )pi n AC D

Nernstian (reversible) systems

• Peak current is linear with square root of scan rate

• No effects of scan rate on peak potential

• Reductive peak current is equal to oxidative peak current

• Value of peak potential difference is 58 mV/n

• / 2 2.2p p

RTE E

nF

Totally irreversible systems

5 1/ 2 * 1/ 2 1/ 2(2.99 10 ) ( )pi n n AC D

1/ 21/ 20`

00.780 ln lnp

n FRT DE E

n F k RT

Voltammogram and Rate constant

The figure below shows a series of voltammograms recorded at a single voltage sweep rate for different values of the reduction rate constant (kred)

Voltammogram and reverbilitity

The figure below shows the voltammograms for a quasi-reversible reaction for different values of the reduction and oxidation rate constants.

Reversal techniques for the reduction

• If Eλ is at least 35/n mV past the cathodic peak, the reversal peaks all have the same general shapes.

• If the cathodic sweep is stopped and the current is allowed to decay to zero, the resulting anodic i-E curve is identical in shape to the cathodic one, but is plotted in the opposite direction on both the I and E axes.

Multicomponent systems (1)

• For a two-component system this technique allows establishing the baseline for the second wave by halting the scan somewhere before the foot of the second wave and recording the i-t curve, and then repeating the experiment.

• The second run is made at the same rate and continues beyond the second peak.

Multicomponent systems (2)

• For a two-component system, an alternate experimental approach involves stopping the sweep beyond Ep and allowing the current to decay to a small value (the concentration gradient of O is essentially zero near the electrode).

• Then one continues the scan and measures ip′ from the potential axis as a baseline.

Multistep charge transfers

• For the stepwise reduction of a substance O, the situation is similar but more complicated.

• In general the nature of the i-E curve depends on △E0= E0

2-E01.

• When △ E0 <-100 mV, two separate waves are observed. When △ E0 is between 0 and -100 mV, the individual waves are merged into a broad wave. When △ E0 =0, a single peak with a peak current intermediate between those of those of single-step 1e and 2e reactions is found. For △ E0 ≥180 mV, a single wave characteristic of a direct 2e reduction is observed.

Electrode reactions with coupled homogeneous chemical reactions

• If E represents an electron transfer at the electrode surface, and C represents a homogeneous chemical reaction.

• Classification of reactions: CE reaction, EC reaction, Catalytic (EC′) reaction, ECE reaction.

Notes

• kf: heterogeneous rate constant for oxidation

• kb: heterogeous rate constant for reduction

• K: equilibrium constant

• λ: dimensionless homogeneous kinetic parameter, specific to mechanism

• DP: diffusion zone, KP: pure kinetic region,

Following reaction-EC

• Note that at small values of λ,essentially reversible behavior is found. For large values of λ (in the KP region), no current is observed on scan reversal and the shape of the curve is similar to that of a totally irreversible charge transfer.

• In the KP region, Ep is given by

1/ 2 0.780 ln2p

RT RTE E

nF nF ( )

knF

RT

The figure below shows a cyclic voltammogram recorded for the EC reaction when the chemical rate constant kEC is extremely large.

EC' mechanism

2-hydroxyacridinone

• Electrochemical oxidation of 2-hydroxyacridinone was studied by cyclic voltammetry (CV), spectro-electrochemical methods and controlled potential electrolysis. The photochemical oxidation was also investigated.

Z. Mazerska, S. Zamponi, R. Marassi, P. Sowiński, J. Konopa. J. Electroanal. Chem. 521 (2002) 144–154

Voltammograms

• Voltammograms were obtained at a glassy-carbon electrode (area: 0.7 cm2). A conventional three-electrode electrochemical cell containing a platinum counter electrode (CE) and a saturated calomel reference electrode (SCE) was employed. All samples were deoxygenated by passing Ar for 10 min. The electrodes were cleaned between runs by polishing with Al2O3 suspension (0.05 μM).

Voltammograms

• On the first positive sweep one oxidation peak, Ia, appeared and three significantly lower peaks, Ic, IIc and IIIc, were formed in the reverse scan. On the second positive sweep new oxidation bands, IIIa and IIa, were observed, which seem to form couples with the reduction peaks, IIIc and IIc, respectively. The cyclic voltammograms recorded under various pH conditions are presented.

Photochemical synthesis

• The 1 mM solution of 2-hydroxyacridinone in the quartz flask was exposed to the light emitted with the UV lamp and was stirred intensively during the respective period of time.

• It is demonstrated, by comparison with the voltammogram of the substrate, that photochemical product p2 was the species responsible for the IIIc–IIIa couple.

Adsorbed intermediates in electrode processes

• Only adsorbed O and R electro-active-nernstian reaction:

• Only adsorbed O electroactive-irreversible reaction:

2 2*

4p

n FI A

RT

00` ln( )p

RT RTkE E

n F n F

Electrochemical behavior of riboflavin immobilized on different matrices

A.C. Pereira, A.S. Santos, L. T. Kubota. J. Colloid Interface Science 265 (2003) 351–358.

Effects of Scan rate on voltammograms

Effects of Scan rate on voltammograms

Cyclic voltammograms of the eletrostaticallyassembled iron porphyrin ITO modified electrode in an aqueous solution containing o.1 mol/L trifluoromethanesulphonate lithium

Structural representation of meso-tetra(4-pyridyl) porphynato iron(III)

Cyclic voltammograms of the NADH solutions using (A) a bare glassy carbon electrode and (B) an electrode modified with tetraruthenated cobalt porphyrin

Structural representation of the tetraruthenated cobalt porphyrin complex

Cyclic voltammograms of the tetraruthenated cobalt porphyrin complex (A) and (B) the corresponding films

Multiclyclic voltammogram of [Ru(tpp)(bpy)2] (tpp= 5,10,15,20-tetraphenylporphyrin) at scan rate of 0.2 V/s in 0.1 mol/L TBAP-dichrolomethane

Cyclic voltammograms of the poly-[Ru(tpp)(bpy)2] (tpp= 5,10,15,20-tetraphenylporphyrin) deposited on the platium electrode in 0.1 mol/L TBAP-dichrolomethane, scan rate of (a) 100, (b) 80, (c) 60, (d) 40, (e) 20 mV/s